The present invention relates to an electrical stimulator adapted to stimulate bodily tissue and more particularly to an electrical stimulator having a plurality of stimulation channels adapted to stimulate bodily tissue.
Electrical stimulators adapted to stimulate bodily tissue are well known. Examples of such electrical stimulators include cochlear implants and transcutaneous electrical nerve stimulators. A cochlear implant supplies an electrical current to electrically stimulate the auditory nerve in order to simulate hearing in an otherwise deaf individual. Transcutaneous electrical nerve stimulators (TENS) are utilized for pain control or for controlled muscle activation. In both the cochlear implant and the TENS stimulators, a pair of electrodes are attached to the bodily tissue to be stimulated. Electrical current is then supplied to this electrode pair to provide a stimulation current between the electrodes which passes through the bodily tissue to be stimulated. This electrical current in the bodily tissue stimulates the appropriate nerves, i.e., the auditory nerve for the cochlear implant and pain bearing nerves for the TENS, to achieve the desired function, i.e., simulated hearing or alleviation of pain, respectively.
In certain situations, it is desirable to have an electrical stimulator which has a plurality of channels. The plurality of channels may be designed to provide more than one type of information to the bodily tissue to be stimulated. With a cochlear implant, a plurality of channels may supply different types of information. As an example, one channel may provide information about a specific frequency range and a second channel may provide information about a different frequency range. This type of cochlear implant is designed to take advantage of the frequency place value relative to a position within the cochlea. As an example, a TENS stimulator may control different nerves and hence different muscles with different channels.
Usually the theory of operation of such multichannel electrical stimulators is that each channel of the stimulator is completely independent of the others. In practice this may not be the case. The effectiveness of multichannel operation of electrical stimulators is impeded by diversion of current intended to pass between electrodes of one electrode pair to the electrode or electrodes of another electrode pair. Electrical current which is intended to pass between one electrode pair may be diverted to another electrode pair by the conduction of the bodily tissue. In general, attempts to control the interaction between the electrode pairs is accomplished by the physical spacing of the electrode pairs. However, spacing between electrode pairs cannot always be controlled. With a cochlear implant, for example, pairs of electrodes must be rather closely spaced to enable placement of more than one electrode pair within the cochlea.
In essence, the effect is that a plurality of channels of stimulation in an electrical stimulator are not completely isolated from each other. That is, the stimulation of one electrode pair has an effect upon the stimulation of another electrode pair. Thus, less than ideal multichannel operation is achieved. The theoretical result of multichannel stimulation is significantly compromised.
Another mechanism which has been used be achieve the isolation of multiple channels in a multichannel stimulator is to electrically isolate the output stages of the stimulator. This, however, requires rather complex circuitry, with attendant increased cost and decreased reliability, and, in the case where the current is being inductively coupled from external transmitter to an implanted electrode pair as in a typical cochlear implant, a plurality of receiving coils.